Have you looked at any of the email newsletter sponsor marketplaces? I'm asking as your sponsor prices seem a bit low (I would guess you could probably get nearly double what you are).
I did awhile ago and found that many of them charged on the publisher side (and a hefty % at that) so I was instantly turned off. I have enjoyed working with sponsors who read the newsletter themselves too. It hasn't been too much work (only publishing twice per week). My main concern when raising prices is always if that will create a gap in sponsors. I'd rather have every slot booked at a lower price than have gaps…
One way to increase your fees is to keep pricing the same for existing advertisers for a while and charge a higher rate to new advertisers. As you fill in spots with higher priced ads you can then transition older customers to the new rates.